A write strategy of an optical disc writer, and a driving signal of a pickup head to control a laser output power are closely related to the write quality of the recorded optical disc.
FIG. 1 depicts a diagram showing a write strategy of an optical disc writer in the prior art. Data formed on a spiral track of an optical disc comprises a plurality of marks (pits) and non-marks (lands). For forming a mark 10 on a track of an optical disc, a laser beam having a write power Pw is outputted from a laser diode of a pickup head, and is focused on the track of the optical disc. However, for heating the track in a relatively short time and for forming a more precise mark 10, an overdrive power Po must be superposed to the write power Pw in an initial period of forming the mark 10. In another word, if the overdrive power Po can be efficiently controlled to an accurate value, the jitter value of the recorded data can be reduced, so as the write quality of the recorded optical disc can be enhanced. Moreover, as depicted in FIG. 1, the read power Pr, outputted from the laser diode of the pickup head, is for reading the marks and non-marks on the tracks of the optical disc.
Before the data writing process of an optical disc writer, the write power Pw must be determined first via the optical disc writer executing an optimum power calibration in a power calibration area of the optical disc. After the write power Pw is determined, the write power Pw is maintained within a specific range via a close loop control in the optical disc writer.
FIG. 2 depicts a block diagram showing the configuration of a prior-art laser power control system. Generally, the optical disc writer uses a digital write power value to control the laser diode to output a write power Pw. Initially, the digital write power value is applied to the laser power control system and converted to an analog signal via a first digital-to-analog converter DAC 111. Then a compensator 12 generates a compensating signal according to the difference of the analog signal and a feedback signal, and outputs the compensating signal to a first laser driver LDD1 14. According to the compensating signal, the first laser driver LDD1 14 generates a first driving current I1. The first driving current I1 is then outputted to a laser diode LD 16 through a first channel CH1 for driving the laser diode LD 16 to output a predefined write power Pw. For maintaining the write power Pw within a specific range, there is a front monitor diode FMD 18 to detect the laser beam outputted from the laser diode LD 16, and then generates the feedback signal according to the power of the detected laser beam.
Moreover, for making the laser diode LD 16 capable of outputting an overdrive power Po, and to be superposed to the write power Pw within a specific period, a digital overdrive power value is provided. As depicted in FIG. 2, the digital overdrive power value is transferred to a second digital-to-analog converter DAC2 20, and converted to an analog signal to the second laser driver LDD2 22. The second laser driver LDD2 22 then outputs a second driving current I2 within a specific period according to the analog signal outputted from the second digital-to-analog converter DAC2 20. The second driving current 12 is then superposed to the laser diode LD 16 through a second channel CH2 for driving the laser diode LD 16 to output the predefined overdrive power Po.
Because the overdrive power Po is only outputted within a specific period and the period is relatively short, it is hard to maintain the overdrive power Po within a specific range via an open loop control in the optical disc writer. As depicted in FIG. 2, the prior-art optical disc writer uses the second laser driver LDD2 22 and the digital overdrive power value to control the second driving current I2 via an open loop control. That means the optical disc writer superposes the second driving current I2 to the first driving current I1 and outputs the summation current of the second driving current I2 and the first driving current I1 to drive the laser diode LD 16 in the specific period of forming the marks.
It is well understood that the temperature of the optical disc writer is gradually increasing during data writing process, and the laser power outputted from the laser diode is gradually decreasing if a fixed driving current is provided. Because the value of the first driving current I1 can be increased according to the increasing temperature due to the first driving current I1 is controlled by a close loop control, the write power Pw can be maintained within a specific range and is independent of the temperature changes. However, the overdrive power Po cannot be adjusted according to the change of the temperature due to the second driving current I2 is controlled by an open loop control. Therefore, the overdrive power Po outputted from the laser diode will gradually decrease along with the increasing temperature. Because the overdrive power Po cannot be maintained within a specific range, there is a potential that the jitter value may be increased, the write quality may be poor, and the data writing or data reading process may be even failed.